Try a phosphate absorbent filter medium first and hold off on the water changes while the pad does its work. Most likely your tap water has enough phosphates, etc., to feed the algae. Wouldn't have guessed that before, but since increasing water changes has increased your algae, it's a possibility. I don't know if carbon absorbs phosphate but I do know there are specific blocks, sponges, etc. that do suck it right up. And that should starve your algae without hurting anything else.